New types of display systems have recently been developed which can provide capabilities of heads-up viewing, virtual imaging from a compact system, and fully animated computer-generated holographic imaging. Unfortunately, all of these systems are monochrome and have been designed with a single advanced display feature in mind.
The most common type of heads-up display is currently being used on jet fighters (and even in some automobiles) to provide a means of overlaying portions of the instrument panel onto the user's normal visual field. Typically the user observes his surroundings by looking through a holographic optical element (HOE) mounted near the windshield. A very high intensity, taut shadow mask CRT, usually mounted on the ceiling of the aircraft, is used to illuminate the HOE which, in turn, reflects the display onto the eyes of the user. The HOE is designed with optical power such that a virtual image of the graphics display is formed in front of the user. The Bragg angle of the HOE is selected such that the particular wavelength of light emitted by the CRT is reflected toward the user, while at all other wavelengths the HOE acts as a transparent window. The projected image from the CRT appears to overlay the input scene and thus allows the pilot to fly more proficiently.
These types of HOE are typically fabricated from dichromated gelatin (DCG), and can be designed to have extremely high diffraction efficiency (&gt;99%), while remaining nearly perfectly clear (&lt;1% absorptive) at other wavelengths in the visible spectrum. The disadvantages of DCG HOEs are that the gelatin layer can shrink over time and that, in conditions of high humidity, the HOE can become fogged. Recently du Pont has introduced several different types of organic photopolymers which can be used to produce HOEs that retain all of the advantages of DCG but are much easier to fabricate, do not shrink, and are not degraded by humidity.
Although, from an engineering standpoint, this type of display technology is quite mature, it does have a number of shortcomings. Implementation of a color display would be very difficult using this scheme since the three color bands reflected toward the user would be removed from the input scene in transmission. In addition, representation of 3D imagery via real-time computer-generated holography is clearly impossible due to resolution limits imposed by the CRT.
Helmet-mounted displays (HMD) are similar in function to heads-up displays, but they allow the user to see the displayed information regardless of how the user moves his head. Most versions of the HMD are used exclusively to present 3D virtual reality visualization sequences for use in flight simulators. Such systems are being developed to train both military pilots as well as astronauts. These systems operate by utilizing a pair of liquid crystal light valves (LCLV) to protect the imagery through optical fiber bundles to transfer high resolution stereo imagery to the user via helmet-mounted optics. Although such HMD displays provide excellent resolution and stereo vision capability, the electronic support system required to implement this type of display system is quite large.
One of the most interesting implementations of an HMD is manufactured by Reflection Technologies and is now available commercially. A simple and compact architecture is used to implement a 12-inch monocular virtual display which appears to float in front of the user. The virtual screen can be placed at distances from nine inches to infinity by adjusting an intermediate focusing lens. By adjusting this distance to match that of the surroundings, the virtual screen observed by the left eye and the surroundings observed by the right eye will merge by a process called fusion. The system employs conventional optics to meet most of the power, size, and weight requirements of an embedded microsystem. A vertically-mounted array of light-emitting diodes (LEDs) is used to sequentially display columns of the output image. A voice coil actuated scanning mirror is used to sweep the image of the LED array across the visual field to form the output image. Application specific integrated circuits (ASICs) and flexible printed circuit material enable the system to fit into its 1.1.times.1.2.times.3.2-inch, battery-operated package, requiring only 1/3-watt of power. However, the display has some shortcomings, namely that it is monochrome and has only 720.times.280 pixel resolution. In its current implementation, the system is designed to interface with IBM compatible personal computers via a computer graphics adapter (CGA). Although somewhat limited in capability, the Reflection Technologies design is the most practical system developed to date for application to an embedded microsystem.
Recently the MIT Media Laboratory demonstrated a high resolution optical scanning system which has the capability of displaying computer-generated holographic images at video frame rates. The scanner architecture utilizes a tellurium dioxide (TeO.sub.2) AO Bragg cell in conjunction with a rotating polygon mirror to produce a scanned output image with extremely high horizontal resolution. Collimated input light from a 10 mW helium neon laser is used to illuminate a Bragg cell, which, in response to a 50-MHz real-time video input signal, proportionally diffracts light into a spatially-varying first-order output. The output from the Bragg cell is focused onto an 18-sided rotating polygon scanning mirror, which spins in the opposite direction of acoustic propagation to exactly cancel the acoustic fringe pattern traveling through the cell. The overall effect is a synthetic aperture with a time bandwidth product (TBWP) equal to the product of the number of sides on the polygon times the TBWP of the cell (18.times.40 .mu.s.times.50 MHz=32,000 resolvable points). An oscillating scan mirror is used to position each horizontally-oriented 720-.mu.s synthetic aperture to the correct position in the output plane. Since a 40-MHz frame rate was utilized (25 ms/frame), the vertical resolution of the display is limited to just 25 ms/(18.times.40 .mu.s)=35 lines. It has been asserted that vertical resolution can be sacrificed because most depth information is perceived from a pair of horizontally-oriented eyes. It seems clear, however, that future holographic display systems must have increased vertical resolution to be practical. Disadvantages of the MIT approach are: (1) it requires a moving mirror, (2) the image is limited to monochrome, (3) real world scenes cannot be superpositioned with the virtual image, and (4) a high-bandwidth data pipe is needed to play the image in real-time.
The principal difference between a high-resolution display and a holographic display is that the holographic display must possess sufficient spatial bandwidth to reproduce the complex light interference patterns originating from an object. This implies pixel spacing of 1 .mu.m or less. For a 3-cm.times.3-cm square display, this would result in 30K.times.30K=900M elements. Adding video frame rates and color would generate more than 81 GB/s of data throughput. Applying standard JPEG near-lossless compression would reduce this to about 8.1 GB/s--still much higher than most supercomputers can generate today.
Personal virtual display systems now available, or being studied in various research laboratories, suffer from poor resolution, are constructed using many moving parts, and are difficult to implement in full color. In addition, currently-available holographic display systems require enormous communication channel bandwidth and have large power demands. Furthermore, many of the conventional display technologies commonly used today (CRT, LCD, plasma) are not easily extendable to support holographic resolution and full color. Thus, a new optical display approach is required that suffers from none of these disadvantages.